1. Field of the Invention
The present invention relates in general to predistortion linearized communication systems and related methods. More particularly, the invention is directed to transmitters equipped with narrow bandwidth band-pass filters for terrestrial communication networks.
2. Description of the Prior Art and Related Background Information
Wireless network operators are facing ever shrinking spectrum challenges. In high density markets such as that found in larger urban environments, every available kHz of available spectrum has been allocated and used to provide voice and data traffic between mobile user equipment (“UE”) and the Base Station (“BS”). To increase wireless capacity, network operators are forced to add carriers and/or split cell sites but eventually are reaching interference limits or are unable to find a suitable location where another BS and its associated equipment (e.g., tower, equipment, antennas, etc) can be installed. Further complicating and limiting the amount of useful spectrum are regulatory interference requirements placed on wireless network operators. Regulatory requirements, such as those mandated by the FCC, stipulate the amount of harmful interference that can be tolerated from BS transmitters operating within its assigned frequency allocation to nearby communication services. In most cases, detected levels of harmful interference levels caused by BS transmitters in nearby communication spectrum must be substantially attenuated and not exceed prescribed levels. To keep the amount of harmful interference from spilling into adjacent communication bands, BS transmitters usually employ highly linear transmitters which keep distortion products at a minimum. Highly linear transmitters utilize power amplifiers which must maintain as linear operation as possible, and the power amplifier is designed to operate within its linear region given the range of possible input signal amplitudes. However if the input signal has an amplitude which causes the power amplifier to operate outside the linear region, the power amplifier introduces nonlinear components or distortion to the signal. Generally, power amplifiers are characterized as having a compression threshold, and input signals having amplitudes above compression threshold are clipped at the amplifier output. In addition to distorting the amplified input signal, the clipping or nonlinear amplification of the input signal generates spectral regrowth which can interfere with communication services in adjacent frequency bands. The problem of non-linear distortion is very common in wireless communications systems that provide high power amplification of transmit signals with very large peak to average power ratios (“PAR”). In one example of large PAR signals of a code division multiple access (“CDMA”) system, a single 1.25 MHz wide carrier can typically have a PAR of 11.3 dB. In another example orthogonal frequency division multiplexing (“OFDM”), multicarrier signals can have a PAR of up to 20 dB.
Unfortunately, the efficiency of the BS amplifier is inversely related to its linearity. To achieve a higher degree of linearity, the amplifiers are biased to operate in the class AB. Numerous techniques and amplifier topologies are used to maximize amplifier RF to DC efficiency, but linearity requirements mandated by modern wireless communication systems dictate the use of class AB modes or combination of AB and C. Consequently, a significant portion of DC power is dissipated by the amplifiers as heat which must be removed. Typically, BS amplifiers use heat sinks and fans to remove heat from RF power devices which further add cost, size, and weight to the base station equipment. Thus, there has been a great deal of effort to reduce the amount of heat generated by BS power amplifiers in a quest to improve amplifier efficiency without degrading amplifier linearity.
When a power amplifier (“PA”) is operated with CDMA (or similar signals) at its input, the PA will amplify the desired signal as well as generate unwanted intermodulation (“IM”) products. These IM products increase rapidly as the PA output is driven to its saturation point. To achieve the desired linearity at the PA output (without predistortion), the PA must be operated at backoff output power level from its saturation point (PSAT3dB). Unfortunately PA operation at backoff power levels limits PA's maximum useful output power level so that the full range of output signal dynamic range is well within the linear region of the PA transfer curve. However, such operating point negatively impacts the PA's efficiency. Efficiencies of 10% or less for conventionally constructed Class AB PAs are not unusual when operated with input signals having 8 to 9 dB peak-to-average ratio PAR while only marginally meeting system linearity requirements.
In view of recent developments in Digital Pre-Distortion (“DPD”), DPD has become a linearization method of choice for Class NAB PAs transmitting 60 W average power and below. A DPD linearization approach lends itself in solving several previously un-attainable performance limitations such as exhibiting high efficiency with good linearity. These improvements stem from DPD operating point that allows PA's to operate close or even slightly above its PSAT during peak signal transition. DPD usually use techniques where a correction signal is created and amplified along with input signal through the PA's in order to reduce the overall distortion at the output of the PA. A DPD can be an optimized CDMA signal (such as IS-95) which tends to have with a large PAR 9.7 dB (0.01% probability on the CCDF) for a single carrier CDMA with pilot, paging, sync and 6 traffic channels (Walsh codes 8-13). A single channel IS-95 has channel bandwidth 1.23 MHz and DPD generally is optimized to reduce third-order IM products. In such application, DPD can predistort PA so that resultant Adjacent Channel Leakage Ratio (“ACLR”) of 48 to 50 dBc at 885-kHz offset, which is typically 14 dB or better over PA ACLR performance operating without predistortion. Similarly, a DPD can be also be optimized to provide cancellation performance for a four carrier WCDMA input signal over a 20 MHz signal band. The DPD performance is usually hampered by the memory effects of the PA which potentially limit DPD effectiveness. The memory effects in a PA amplifier stage are defined as a change of the amplitude and phase in distortion components due to the previous signals. However, if the PA is designed with bias circuits that tend to reduce or limit memory effects, DPD can typically provide linearization for a four-carrier WCDMA signal that results in ACLR of 46 to 48 dBc range with 13 dB or better cancellation at 5-MHz offset. In most applications, PA efficiencies above 25 percent for Class AB PAs and to 40 percent or more for Doherty type PAs is achievable.
Given that DPD linearization can provide ACLR performance additional attenuation of IM products into adjacent spectrum can be achieved by providing a bandpass filter or in case of FDD system duplexer can be implemented in the output of the RRH.
In a typical FDD implementation, a cavity duplexer provides isolation between receiver and transmitter of the RRH as well as adequate degree of TX IM rejection into adjacent spectrum. Since a duplexer is generally required for FDD RRH, its inclusion introduces a host of issues, such as additional output insertion loss and filter pass band response ripple on the transmitted signal quality. The latter manifests itself in an operational situation where CDMA carrier is positioned close to filter roll-off or a band edge. This usually takes place when a network operator configures a CDMA (or WCDMA) carrier in its allocated frequency spectrum in close proximity to the band edge. When such carrier frequency is selected, the net effect of the filter roll-off characteristic impacts CDMA carrier flatness which in turn degrades Rho (“ρ”). The Rho of the BS is figure of merit which is a measure of modulation quality of the transmitted CDMA signal. A Rho of 1.0 is associated with an ideal transmitted signal, essentially all of the power in the CDMA carrier is being transmitted correctly without any multipath. The CDMA standard calls for Rho>0.912 and, in practice, BS transmitter measurements Rho>0.94 indicate a normal BS transmitter operation.
For BS operating W-CDMA air standard (per 3GPP) BS transmitter quality can be summarized in the table below:
3GPP Requirements3GPP Requirement LimitsEVM17.5%PCDE−33 dBACLR1  45 dBACLR2  50 dB
The Error Vector Magnitude is a measure of the difference between the reference waveform and the measured waveform. This difference is called the error vector. Both waveforms pass through a matched Root Raised Cosine filter with bandwidth 3.84 MHz and roll-off α=0.22. Both waveforms are then further modified by selecting the frequency, absolute phase, absolute amplitude and chip clock timing so as to minimize the error vector. The EVM result is defined as the square root of the ratio of the mean error vector power to the mean reference power expressed as a percentage (“%”). The EVM measurement in the test set compares the received signal's IQ modulation characteristics to an ideal signal, as defined in 3GPP TS 34.121 section 5.13 and annex B.
Introduction of a narrow bandpass filter in the output of the BS PA results in CDMA carrier flatness degradation especially if the carrier is positioned close to filter roll off. Typical narrow band pass filters also tend to exhibit rapid group delay change near filter roll-off. The combination of amplitude and group delay variation introduced by the narrow band pass filter degrades Rho and EVM parameters.
Accordingly, a need exists to improve performance in communication systems.